---
layout: "default"
title: "LazyFilterIndex"
description: "Swift documentation for 'LazyFilterIndex': The Index used for subscripting a LazyFilterCollection."
keywords: "LazyFilterIndex,struct,swift,documentation,advancedBy,advancedBy,distanceTo,successor,base"
root: "/v2.1"
---

<div class="intro-declaration"><code class="language-swift">struct LazyFilterIndex&lt;BaseElements : CollectionType&gt;</code></div>

<div class="discussion comment">
    <p>The <code>Index</code> used for subscripting a <code>LazyFilterCollection</code>.</p>

<p>The positions of a <code>LazyFilterIndex</code> correspond to those positions
<code>p</code> in its underlying collection <code>c</code> such that <code>c[p]</code>
satisfies the predicate with which the <code>LazyFilterIndex</code> was
initialized.</p>

<p><strong>Note:</strong> The performance of advancing a <code>LazyFilterIndex</code>
  depends on how sparsely the filtering predicate is satisfied,
  and may not offer the usual performance given by models of
  <code>ForwardIndexType</code>.</p>
</div>

<table class="standard">
<tr>
<th id="inheritance">Inheritance</th>
<td>
<code class="inherits">Equatable, ForwardIndexType, _Incrementable</code>
<span class="viz"><a href="hierarchy/">View Protocol Hierarchy &rarr;</a></span>
</td>
</tr>



<tr>
<th>Import</th>
<td><code class="language-swift">import Swift</code></td>
</tr>

</table>




<h3>Instance Variables</h3>
<div class="declaration" id="var-base_-baseelements-index">
<a class="toggle-link" data-toggle="collapse" href="#comment-var-base_-baseelements-index">var base: BaseElements.Index</a><div class="comment collapse" id="comment-var-base_-baseelements-index"><div class="p">
    <p>The position corresponding to <code>self</code> in the underlying collection.</p>

    <h4>Declaration</h4>    
    <code class="language-swift">var base: BaseElements.Index { get }</code>

    </div></div>
</div>



<h3>Instance Methods</h3>
<div class="declaration inherited" id="func-advancedby_">
<a class="toggle-link" data-toggle="collapse" href="#comment-func-advancedby_">func advancedBy(<wbr>_:)</a>
        
<div class="comment collapse" id="comment-func-advancedby_"><div class="p">
    <p>Return the result of advancing <code>self</code> by <code>n</code> positions.</p>

<p><strong>Returns:</strong>
  - If <code>n &gt; 0</code>, the result of applying <code>successor</code> to <code>self</code> <code>n</code> times.
  - If <code>n &lt; 0</code>, the result of applying <code>predecessor</code> to <code>self</code> <code>-n</code> times.
  - Otherwise, <code>self</code>.</p>

<p><strong>Requires:</strong> <code>n &gt;= 0</code> if only conforming to <code>ForwardIndexType</code>
<strong>Complexity:</strong>
  - O(1) if conforming to <code>RandomAccessIndexType</code>
  - O(<code>abs(n)</code>) otherwise</p>

    <h4>Declaration</h4>    
    <code class="language-swift">func advancedBy(n: LazyFilterIndex&lt;BaseElements&gt;.Distance) -&gt; LazyFilterIndex&lt;BaseElements&gt;</code>
    
        <h4>Declared In</h4>
        <a href="../../protocol/ForwardIndexType/"><code>ForwardIndexType</code></a>    
</div></div>
</div>
<div class="declaration inherited" id="func-advancedby_limit_">
<a class="toggle-link" data-toggle="collapse" href="#comment-func-advancedby_limit_">func advancedBy(<wbr>_:<wbr>limit:)</a>
        
<div class="comment collapse" id="comment-func-advancedby_limit_"><div class="p">
    <p>Return the result of advancing <code>self</code> by <code>n</code> positions, or until it
equals <code>limit</code>.</p>

<p><strong>Returns:</strong>
  - If <code>n &gt; 0</code>, the result of applying <code>successor</code> to <code>self</code> <code>n</code> times
    but not past <code>limit</code>.
  - If <code>n &lt; 0</code>, the result of applying <code>predecessor</code> to <code>self</code> <code>-n</code> times
    but not past <code>limit</code>.
  - Otherwise, <code>self</code>.</p>

<p><strong>Requires:</strong> <code>n &gt;= 0</code> if only conforming to <code>ForwardIndexType</code>.</p>

<p><strong>Complexity:</strong>
  - O(1) if conforming to <code>RandomAccessIndexType</code>
  - O(<code>abs(n)</code>) otherwise</p>

    <h4>Declaration</h4>    
    <code class="language-swift">func advancedBy(n: LazyFilterIndex&lt;BaseElements&gt;.Distance, limit: LazyFilterIndex&lt;BaseElements&gt;) -&gt; LazyFilterIndex&lt;BaseElements&gt;</code>
    
        <h4>Declared In</h4>
        <a href="../../protocol/ForwardIndexType/"><code>ForwardIndexType</code></a>    
</div></div>
</div>
<div class="declaration inherited" id="func-distanceto_">
<a class="toggle-link" data-toggle="collapse" href="#comment-func-distanceto_">func distanceTo(<wbr>_:)</a>
        
<div class="comment collapse" id="comment-func-distanceto_"><div class="p">
    <p>Measure the distance between <code>self</code> and <code>end</code>.</p>

<p><strong>Requires:</strong>
  - <code>start</code> and <code>end</code> are part of the same sequence when conforming to
    <code>RandomAccessSequenceType</code>.
  - <code>end</code> is reachable from <code>self</code> by incrementation otherwise.</p>

<p><strong>Complexity:</strong>
  - O(1) if conforming to <code>RandomAccessIndexType</code>
  - O(<code>n</code>) otherwise, where <code>n</code> is the function&#39;s result.</p>

    <h4>Declaration</h4>    
    <code class="language-swift">func distanceTo(end: LazyFilterIndex&lt;BaseElements&gt;) -&gt; LazyFilterIndex&lt;BaseElements&gt;.Distance</code>
    
        <h4>Declared In</h4>
        <a href="../../protocol/ForwardIndexType/"><code>ForwardIndexType</code></a>    
</div></div>
</div>
<div class="declaration" id="func-successor">
<a class="toggle-link" data-toggle="collapse" href="#comment-func-successor">func successor()</a>
        
<div class="comment collapse" id="comment-func-successor"><div class="p">
    <p>Returns the next consecutive value after <code>self</code>.</p>

<p><strong>Requires:</strong> The next value is representable.</p>

<p><strong>Complexity:</strong> Amortized O(M), where M is the average distance in
  the base collection between elements that satisfy the
  predicate.</p>

<p><strong>Note:</strong> this operation may not satisfy the expected complexity
  for models of <code>ForwardIndexType</code>.</p>

    <h4>Declaration</h4>    
    <code class="language-swift">func successor() -&gt; LazyFilterIndex&lt;BaseElements&gt;</code>
    
    
</div></div>
</div>


